The voice might take you aback if you've seen Cooper in any other project, ever. That is just not the way he sounds. As a result, Cooper's baritone in the highly hyped film, released Friday, is getting quite a bit of attention.

And yes, if you're wondering - it was extremely painful to transform his voice like that.

"It hurt my esophagus," Cooper told The Washington Post's Sonia Rao in a recent interview. "I would have pain for the first couple of months. It really hurt."

Luckily, Cooper said, he had an ace dialect coach named Tim Monich who created a detailed plan to figure out the exact accent and rhythm.

"The voice is everything. It all begins and ends with the voice, as an actor - and as a human being," said Cooper, who also directed the film. "That's why singing is such a wonderful tool to open up your body and soul, because you can't hide. I knew I wanted to lower my voice."

Monich traveled to Los Angeles and worked with Cooper for about six months, five days a week. He had a library of accents from interviews that he's collected over 40 years with people from all over the country: truck stops, restaurants, you name it.

There was a bit of trial and error at first, Cooper said. At first, his character sounded very country with a twang. But Cooper decided it didn't "feel right" to go all-out country. Soon, they landed on the perfect hybrid: actor Sam Elliott's voice.

Elliott, a Sacramento native with Southern roots, has the exact growl that Cooper craved. Then the challenge was to get Elliott to sign on to the movie, because Cooper wanted him to play his character's brother. And he really needed to impress Elliott with an impression of his own voice.

Thankfully, it worked. Elliott told USA Today that he went to Cooper's house, where the actor talked to him about the role - and then played him a tape of himself, essentially imitating Elliott's voice. Elliott liked what he heard.

"He was sounding a lot like me," Elliott said. "I didn't have any tips at all, he already had it down."

Even though Cooper is back to his regular dialect now, he admits that the whole process of changing his voice was one of the most nerve-wracking parts of the movie.

"People have asked: 'What was the most terrifying thing?' That was, actually. The speaking voice," Cooper said. "I thought, 'I've got to work on it to the point where it becomes a part of me' ... otherwise I look like a joke, you know? After I worked for about six months, it was in me."